Retinal ganglion cells (RGCs) from the ventrotemporal (VT) region of the retina are the source of the projections that course ipsilaterally at the midline optic chiasm. I propose to investigate the cellular and molecular dynamics of a single receptor-ligand pair that has been found to regulate the repulsion of VT axons from the chiasm midline. EphB1 is expressed in VT RGCs and ephrin-B2 in the radial glia at the chiasm. We will use a RGC-chiasm explant co-culture model to determine the subcellular localization of EphB1 on axons and growth cones as well as the spatial and temporal "fate" of this receptor as it interacts with ephrin-B2 at its midline targets. By utilizing EphB-null animals, tagged constructs and immunohistochemistry, I will characterize the general profile as well as several possible mechanisms by which EphB1 protein could be rapidly up and downregulated on axons in response to molecular cues in the midline environment. Experiments will determine upregulation, including the possibility of local translation of EphB1 in growth cones of VT axons in response to ephrin-B2, and will investigate mechanisms for the termination of EphB1- mediated repulsion postchiasmatically, such as endocytosis or proteolytic cleavage. [unreadable] [unreadable]